Will Wright, the person behind the Sims, &c., has a new game concept.
This is the bit that got my attention:
Wright led his tiny microscopic creature around, eating little green cells and avoiding brown cells or black cells that tried to eat him. "This is kinda like Pac-Man," he explained. After he ate enough, he was rewarded with the ability to lay an egg -- which he did.

Clicking on the egg brought up a creature editor, and allowed the player to "evolve" with a new generation of critters. The editor was amazingly flexible. Wright could give his creature extra vertebrae, he could give it fins or tails to move faster, he could add claws or extra mouths, whatever he wanted. More importantly, all the creature animations weren't hard-coded; they were dynamic. If he put six tails on his creature, the game would figure out how a six-tailed creature would move. The critter was completely his.http://www.teencrackwhores.com/comment.php?entry=785
"Lure the players into being creative"

After hatching his next-generation creature (who had a spike on its head), Wright was able to swim around the sludge a little faster and now he could eat the brown cells as well as the green. One step up along the food chain!

You should really keep reading the article, though, because it just gets cooler and cooler. Instead of wasting your upgrades on headspikes, you can increase brain capacity and develope sentience!?!?! And then... yeah, I want to play this now.